Generally, a stack of a fuel cell has a stacked structure of a plurality of cells, in which a cooling plate for cooling the stack (cells) is inserted between each sub-stack composed of a few layers of cells. The cooling plate has a coolant channel inside, through which a coolant flows so as to cool the stack. As herein described, since a coolant for a fuel cell circulates through a stack where electric power is generated, namely, between sub-stacks, high insulation performance is required in order to prevent electric leak to the outside of the stack and lowering of electrical efficiency caused by the resistance in the coolant (reduction of energy loss). To ensure such insulation performance and respond to the demand of retaining cooling efficiency, pure water has been used as a coolant in conventional arts. In addition to such demands, anti-rust properties are required for a coolant for fuel cell stacks in order to maintain a long product life of cooling plates. This requirement has been generally addressed by using a stainless steel material with high anti-rust properties for a cooling plate or by adding iron ions to a coolant as disclosed in JP Patent Publication (Kokai) No. 2-21572 (1989).
However, although such conventional approaches are effective for so-called stationary, installed-type medium or large fuel cells, or constantly operating fuel cells, they are not necessarily effective for non-stationary, small fuel cells such as fuel cells installed in vehicles or intermittently operating fuel cells.
For example, since the temperature of a coolant in intermittently operating non-stationary fuel cells falls to ambient temperature when the cell is not in operation, anti-freezing properties are required for the coolant under conditions where the ambient temperature is the freezing point or lower. This is because, if the coolant freezes, the cooling circuit including a cooling plate may be damaged. Furthermore, when the cooling circuit is damaged, possibly fuel cells do not operate sufficiently.
In this situation, taking anti-freezing properties into account, use of a coolant for cooling an internal combustion engine as a non-freezing coolant is an option. Such a coolant for cooling an internal combustion engine, however, is basically used in the area where no electricity is generated, and therefore low conductivity is not expected, and thus the coolant has extremely high electric conductivity. On the other hand, since electricity flows through a cooling pipe of a fuel cell stack, when the coolant has a high electric conductivity, the electricity generated in the fuel cell flows into the coolant and is lost. For this reason, such a coolant is unsuitable for a coolant for cooling a fuel cell stack.
Moreover, for non-stationary fuel cells installed in vehicles, making a fuel cell system including a cooling circuit lightweight is an important issue to be solved. Therefore, for achieving lightweight, light metals having high thermal conductivity such as aluminum materials are expected to be used for cooling plates or heat exchangers. Such light metals generally do not have anti-rust properties as high as that of stainless steel materials, and therefore, the coolant itself must have anti-rust properties.
Given this, the present inventors have filed an application related to a coolant comprising a base containing water and an anti-rust additive which keeps the conductivity of a coolant low and maintains the hydrogen ion exponent of the coolant at about neutral, which has been published as JP Patent Publication (Kokai) 2001-164244. Examples of such anti-rust additives include weak alkaline additives, weak acidic additives and nonionic substances. Saccharides such as quercetin and nonionic surfactants such as alkylglucoside are disclosed as nonionic substances.